1. Field of the Invention
The invention pertains to the field of optical networks employing dense wavelength division multiplexing (hereafter DWDM) and in particular to a method and apparatus for controlling the length of a laser cavity.
2. Description of the Related Art
The evolution of telecommunications networks has been such that the amount of data that can be carried by a single fiber have in general, greatly increased. Key to transporting large volumes of information over a single fiber is DWDM technology. DWDM enables the transmission of multiple xe2x80x9ccolorsxe2x80x9d or wavelengths of light over a single fiber, thereby greatly enhancing data throughput. The source for each wavelength of light is a single frequency laser, which is tuned to a precise wavelength during manufacture and/or during operation. Transmission lasers may be designed to operate a single wavelength for the duration of their useful life, or may be designed to be xe2x80x9ctunablexe2x80x9d, that is, their wavelength of operation may be changed from time to time.
DWDM systems typically comprise multiple separately modulated laser systems at the transmitter. These laser systems are designed or actively tuned to operate at different wavelengths.
When their emissions are combined in an optical fiber, the resulting WDM optical signal has a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in semiconductor amplifier systems or gain fiber, such as erbium-doped fiber and/or regular fiber, although semiconductor optical amplifiers are also used in some situations.
At the receiving end, the channels are usually separated from each other using, for example, thin film filter systems to thereby enable detection by separate detectors, such as photodiodes.
The advantage of DWDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data in WDM systems is typically less than comparative non-multiplexed systems. This is because optical amplification systems required along the link is shared by all of the separate wavelength channels transmitted in the fiber. With non-multiplexed systems, each channel/fiber would require its own amplification system.
However, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, optical components are required to combine the channels into, and separate the channels from, the WDM optical signal. Moreover, there is the danger of channel drift where the channels lose their spectral separation and overlap each other. This interferes with channel separation and demodulation at the receiving end.
The optical signal generators, e.g., the semiconductor laser systems that generate each of the optical signals corresponding to the optical channels for a fiber link, must have some provision for wavelength control. Especially in systems with center-to-center wavelength channel spacings of less than 1 nanometer (nm), the optical signal generator must have a precisely controlled carrier wavelength. Any wander impairs the demodulation of the wandering signal at the far end receiver since the wavelength is now at a wavelength different than expected by the corresponding optical signal detector, and the wandering signal can impair the demodulation of spectrally adjacent channels when their spectrums overlap each other.
In addition to wavelength stability, optical signal generators that are tunable are also desirable for a number of reasons. First, from the standpoint of manufacturing, a single system can function as the generator for any of the multiple channel wavelength slots, rather than requiring different, channel slot-specific systems to be designed, manufactured, and inventoried for each of the hundreds of wavelength slots in a given WDM system. From the standpoint of the operator, it would be desirable to have the ability to receive some wavelength assignment, then have a generator produce the optical signal carrier signal into that channel assignment on-the-fly.
For telecommunications applications involving DWDM, the wavelength range used is in what is known as the third window. The third window is the spectral region within which the attenuation exhibited by the transmission medium (commonly silica glass) is the lowest. Although loosely defined, the third window may be identified to lie in the spectral region from 1500 nm to 1650 nm. Within this window the designations xe2x80x9cSxe2x80x9d, xe2x80x9cCxe2x80x9d and xe2x80x9cLxe2x80x9d represent subdivisions of this spectral region. An object of transmission laser performance is therefore the capability to address the spectral region associated with S, C and L-band wavelengths. A further object of a transmission laser is that it is compliant with what is known as the xe2x80x9cITU gridxe2x80x9d. The ITU grid is a defined standard covering the placement, in frequency space, of optical channels launched onto a fiber. Transmission lasers must exhibit optical specifications compatible with high performance optical transmission.
For a detailed description of the structure an optical performance requirements set on transmission lasers resort may be had to J. Gowar, xe2x80x9cOptical Communications Systemsxe2x80x9d, Second Edition, Prentice Hall International Series in Optoelectronics, pages 257 to 487, inclusive, the contents of which are incorporated herein by reference.
It is desirable that transmission lasers (tunable or fixed) operate with a single longitudinal mode (Fabry-Perot mode) in the laser cavity, and that the primary longitudinal mode that is lasing does not change over the duration of operation of the laser. xe2x80x9cMode hoppingxe2x80x9d, that is, the changing of the longitudinal mode of operation, may be prevented by carefully controlling the optical length of the laser cavity.
The optical length of the laser cavity is generally a function of the effective index of refraction in the materials in the cavity, and the mechanical length of the cavity. Both of these properties are strong functions of temperature, thus temperature changes are a major source of disturbance that can cause mode hopping. Other physical phenomena that can lead to mode hopping include mechanical stress (causing length changes), vibration, changes in the material index due to aging and the like.
Another desirable feature of transmission lasers is the absolute frequency at which they operate. In order to control the absolute frequency, the optical length of the laser cavity should be controlled such that the desired absolute frequency is coincident with one of the cavity""s longitudinal modes.
One method of controlling cavity length, and thereby preventing mode hopping and controlling the absolute frequency of the Fabry-Perot modes, is through active temperature control of the materials in the laser cavity. One method of controlling the temperature of DWDM semiconductor diode lasers is via a temperature sensing thermistor, a proportional integral derivative (PID) feedback control law, and a thermo-electric cooler (TEC) temperature actuator, although different choices for sensors, control laws, and actuators are clearly possible. Temperature control may be used to maintain a constant effective optical cavity length, for example laser devices using a rare earth ion (such as neodymium) doped into a crystalline host material as the active medium.
Another approach is to use a cavity that is constructed of a combination of materials, some in which the optical length increases with increasing temperature, and others in which the optical length decreases with increasing temperature. The goal is to create a cavity that has an optical length that is constant over the normal temperature range of operation, thus mode hopping does not occur, provided significant temperature gradients do not exist across the laser cavity. U.S. Pat. No. 6,324,204 to Deacon, incorporated herein by reference, describes such a device.
Yet another approach for temperature compensation of optical devices employs passive mechanical temperature compensation. The device is mechanically strained, and then attached to a material that has either a negative coefficient of thermal expansion, or is attached to a bimetallic material. In either case, as the temperature increases, the strain on the optical material is relieved, thereby compensating for the intrinsic optical length increase of the optical material.
The above techniques rely on either active means of controlling the temperature of the materials inside the laser cavity, or on passive mechanical means of compensating for changes in the optical length of the cavity. Both of these techniques require the addition of components above and beyond what is strictly necessary to produce a laser device.
There is therefore the need for a device that maintains a constant cavity length through active controls directly associated with the transmitter which prevents mode hopping and controls the absolute frequency of the Fabry-Perot modes of the transmitter. Such a device has the potential to significantly reduce the manufacturing cost via parts count reduction.
The present invention is capable of producing stable, uncooled, single mode lasers for use in optical communications. The lasers of the present invention may operate at a fixed wavelength throughout its operational life, or may be tuned from time to time in order to operate at other wavelengths.
According to one general aspect of the present invention, an apparatus for controlling the length of a laser cavity comprises a laser diode that is configured to produce a beam of energy, the laser diode has a first end and an output end, the first end being in optical communication with a highly reflective mirror. A wave guide having a receiving end and a transmission end is also provided, with the wave guide being comprised of an electro-optical material, wherein the receiving end is in optical communication with the output end, and the transmission end is in optical communication with an output coupler. A plurality of electrodes are disposed in close proximity along a longitudinal axis of said wave guide is further provided, wherein the voltage on each electrode is independently controlled to alter the index of refraction of the wave guide at a position adjacent each electrode. A polarizer is also provided with respect to the transmission end, with the polarizer configured to attenuate the beam of energy.
According to another general aspect of the present invention, a wave guide for controlling the output of an energy source comprises a body having electro-optical material having an input end, an output end, and a longitudinal axis. A plurality of electromagnetic fields coupled to the body is provided, wherein the index of refraction of the body is altered along said longitudinal axis in relation to each electromagnetic field. A polarizer disposed adjacent the output end is configured to filter unwanted portions of a signal associated with the energy source.
According to yet another general aspect present invention, a transmitter for use with a fiber optic telecommunications network comprises a semi-conductor optical gain means configured to receive an input signal, the optical gain means producing an output signal associated with the input signal. A wave guide means is provided that has an input means in optical communication with the optical gain means, the wave guide means comprising an electro-optical material. A plurality of electrode means in electromagnetic communication with the wave guide means is further provided, wherein each electrode means alters the index of refraction of the wave guide means adjacent each electrode means. A filter means disposed on the wave guide means to filter unwanted portions of a signal associated with the output signal is also provided.
According to still another general aspect of the present invention, an electro-magnetically tuned laser source with an athermal resonator comprises a laser gain medium and an intracavity waveguide segment comprised of electro-optical material optically coupled to the laser gain medium. A feedback means is provided for defining a resonant laser cavity including the gain medium and the intracavity waveguide segment, the feedback means including a plurality of electrode means disposed along the intracavity waveguide segment, each electrode being selectably energized for tuning a frequency of operation of the laser cavity.
According to still another general aspect of the present invention, a method for controlling the output frequency of a laser comprises the steps of providing a laser gain medium and placing an intracavity waveguide segment comprised of electro-optical material in optical communication with the laser gain medium. A feedback means is provided for defining a resonant laser cavity, the resonant laser cavity including the gain medium and the intracavity waveguide segment, the feedback means comprising a plurality of electrodes disposed along the intracavity waveguide segment. The electrodes are selectably energized to tune a frequency of operation of the laser.
According to yet another general aspect of the present invention, computer readable storage media stores code which causes a host processor to control a cavity length of a laser assembly in a telecommunication system. The laser assembly comprising a laser gain medium is optically coupled to a waveguide segment, the waveguide segment comprised of electro-optical material and a plurality of electrodes adjacent a surface of the waveguide segment. The code causes the host processor to receive a required operating frequency of the laser assembly. The code measures an actual operating frequency of the laser assembly and receives data from a sensor, the sensor being configured to measure a temperature of the laser assembly. The code selectably energizes each of the electrodes based on the data from the sensor, such that the cavity length of the waveguide segment is controlled.
According to another general aspect of the present invention, a method for controlling the optical length of a laser cavity is provided which comprises the steps of providing a laser gain medium and placing an intracavity waveguide segment comprised of electro-optical material in optical communication with the laser gain medium. A plurality of electrodes are disposed along a longitudinal axis of the intracavity wave guide segment and a predetermined first set of said electrodes is energized to control the optical length of the laser cavity.
Further objects and advantages of the present invention will appear hereinbelow.